Communication devices such as wireless devices are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or Mobile Stations (MS). Wireless devices are enabled to communicate wirelessly in a cellular communications network or wireless communication network, sometimes also referred to as a cellular radio system, cellular system, or cellular network. The communication may be performed e.g. between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the wireless communications network.
Wireless devices may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The wireless communications network covers a geographical area which may be divided into cell areas, wherein each cell area being served by an access node such as a Base Station (BS), e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. Evolved Node B “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
3GPP LTE
3rd Generation Partnership Project (3GPP) Long-Term Evolution (LTE) represents a project within the third generation partnership project, with an aim to improve the Universal Mobile Telecommunications System (UMTS) standard. The 3GPP LTE radio interface offers high peak data rates, low delays and an increase in spectral efficiencies. The LTE physical layer is designed to achieve higher data rates, and is facilitated by turbo coding/decoding, and higher order modulations, up to 256-Quadrature Amplitude Modulation (QAM). The modulation and coding may be adaptive, and may depend on channel conditions. Orthogonal Frequency Division Multiple Access (OFDMA) may be used for the downlink, while Single Carrier Frequency Division Multiple Access (SC-FDMA) may be used for the uplink. The main advantage of such schemes is that the channel response is flat over a sub-carrier even though the multi-path environment may be frequency selective over the entire bandwidth. This may reduce the complexity involved in equalization, as simple single tap frequency domain equalizers, which may be understood as stateless or memoryless filters, may be used at the receiver. OFDMA allows LTE to achieve its goal of higher data rates, reduced latency and improved capacity/coverage, with reduced costs to the operator. The LTE physical layer supports Hybrid Automatic Repeat reQuest (H-ARQ), power weighting of physical resources, uplink power control, and Multiple-Input Multiple-Output (MIMO).
Duplex Schemes
The LTE ecosystem may support both Frequency Division Duplex (FDD) and Time Division Duplex (TDD). This may enable the operators to exploit both the paired and unpaired spectrum, since LTE may have flexibility in bandwidth, as it may support 6 bandwidths 1.4 MegaHertz (MHz), 3 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz.
Frequency Division Duplex
In the case of FDD operation, there may be two carrier frequencies which are represented in FIG. 1, one for uplink transmission (fuplink) and one for downlink transmission (fdownlink). During each frame, 10 Transmission Time Intervals (TTI) or subframes, there may thus be ten uplink subframes and ten downlink subframes, each represented in FIG. 1 as a box, and uplink and downlink transmission may occur simultaneously within a cell. Isolation between downlink and uplink transmissions may be achieved by transmission/reception filters, known as duplex filters, and a sufficiently large duplex separation in the frequency domain.
Time Division Duplex
In the case of TDD operation, there is a single carrier frequency only and uplink and downlink transmissions are separated in the time domain on a cell basis. For example, FIG. 2 shows an example of a TDD frame where some subframes, each represented in FIG. 2 as a box, are allocated for uplink transmissions and some subframes for downlink transmission, with the switch between downlink and uplink occurring in the special subframe denoted as S. Different asymmetries in terms of the amount of resource that is, subframes allocated for uplink and downlink transmission respectively may be provided through the seven different downlink/uplink configurations as shown in Table 1.
TABLE 1TDD configurations supported in LTEDownlink-to-UplinkUplink-Switch-downlinkpointSubframe numberconfigurationperiodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
Message Sequence Chart of Downlink Transmission
FIG. 3 shows an example of a typical message sequence chart for downlink data transfer in LTE. From the pilot or reference signals received from an eNB at 301, a UE may then at 302 compute the channel estimates, then compute the parameters that may be needed for Channel State Information (CSI) reporting. The CSI report may consist of, for example, channel quality information (CQI), a Precoding Matrix Index (PMI), a Rank Information (RI) etc. At 303, the CSI report is sent to the eNodeB via a feedback channel either Physical Uplink Control Channel (PUCCH), periodic CSI reporting, or Physical Uplink Shared Channel (PUSCH), aperiodic. At 304, the eNodeB scheduler may use this information in choosing the parameters for scheduling of this particular UE. At 305, the eNodeB sends the scheduling parameters to the UE in the downlink control channel called Physical Downlink Control CHannel (PDCCH). However, before sending the PDCCH, the eNode B sends control format indicator information on the Physical Control Format Indicator CHannel (PCFICH), which is a physical channel providing the UEs with information necessary to decode the set of downlink control channels (PDCCH). After that, actual data transfer takes place from eNodeB to the UE at 306, via the Physical Downlink Shared Channel (PDSCH).
Uplink Control Channel
In LTE, the uplink control channel may carry information about Hybrid Automatic Repeat reQuest (HARQ)-ACKnowledgment (ACK) information corresponding to the downlink data transmission, and channel state information. The channel state information may typically consist of a Rank Indicator (RI), CQI, and a PMI. Either the PUCCH or the PUSCH may be used to carry this information. The PUCCH reporting may be periodic, and the periodicity of the PUCCH may be configured by the higher layers, while the PUSCH reporting may be aperiodic. Also, there may be various modes for PUCCH and PUSCH, and, in general, the reporting may depend on the transmission mode, and the formats may be configured via higher layer signaling.
Downlink Control Channel
In LTE, the downlink control channel (PDCCH) may carry information about the scheduling grants. Typically, this may consist of a number of M IMO layers scheduled, transport block sizes, modulation for each codeword, parameters related to HARQ, sub band locations and also PMI corresponding to those sub bands.
Typically, the following information may be transmitted by means of the downlink control information (DCI) format: localized/distributed Virtual Resource Block (VRB) assignment flag, resource block assignment, modulation and coding scheme, HARQ process number, new data indicator, redundancy version, Transmitter Power Control (TPC) command for PUCCH, downlink assignment index, precoding matrix index, and number of layers.
Note that, all DCI formats may not use all the information as shown above. In general, the contents of PDCCH may depend on transmission mode and DCI format.
Carrier Aggregation in LTE
Carrier aggregation (CA) was introduced in Release 10 for LTE and/or LTE Advanced (LTE-A) to increase the bandwidth without any modifications of the baseband. In the case of carrier aggregation, multiple LTE carriers, each with a bandwidth up to 20 MHz may be transmitted in parallel to and/or from the same terminal, thereby allowing for an overall wider bandwidth, and correspondingly higher per-link data rates. In the context of carrier aggregation, each carrier may be referred to as a component carrier as, from a Radio Frequency (RF) point-of-view, the entire set of aggregated carriers may be seen as a single, RF, carrier. Till Release 12, up to 5 LTE/LTE-A component carriers may be aggregated, allowing for transmission bandwidths up to 40 MHz for High-Speed Packet Access (HSPA), and up to 100 MHz for LTE/LTE-A.
A terminal capable of carrier aggregation may receive or transmit simultaneously on multiple component carriers. Aggregated component carriers may not need to be contiguous in the frequency domain. Rather, with respect to the frequency location of the different component carriers, three different cases may be identified: intra-band aggregation with frequency-contiguous component carriers, intra-band aggregation with non-contiguous component carriers, and inter-band aggregation with non-contiguous component carriers.
A terminal capable of carrier aggregation may have one downlink primary component carrier and an associated uplink primary component carrier. In addition, it may have one or several secondary component carriers in each direction. Different terminals may have different carriers as their primary component carrier—that is, the configuration of the primary component carrier may be terminal specific.
Additional TDD Configurations in LTE
From the recent mobile data statistics, in general the traffic is asymmetric. This means that DL traffic may be much heavier than uplink. Hence, in order to support more downlink subframes for LTE-TDD systems, a 3GPP RAN plenary has discussed adding more downlink heavy configurations for the currently existing TDD configurations. The configurations under considerations are 10:0:0 and 9:1:0. Both the configurations are configured along with a standalone carrier as the primary carrier.
Downlink heavy configurations, however, with existing methods, negatively affect the performance of wireless devices, e.g., due to reduced throughput, and result in degraded network communications.